In a high speed digital mobile communication of higher than 50 kb/s, a delay distortion due to selective frequency fading caused by group delay dispersion is a serious problem. An equalizer cancels the distortion by using a delay element and a variable weight circuit.
Conventionally, an equalizer operates in two operational modes. The first mode is a training mode which initializes a plurality of variable weight circuits by using a known signal called a training signal. The second mode is a tracking mode which equalizes a receive signal, and updates finely the variable weight circuits following the change of the characteristics of a propagation path. Some conventional methods for a tracking mode are LMS (Least Mean Squares) algorithm, and RLS (Recursive Least Squares) algorithm. Those methods are described in "Adaptive Transversal Filter using Gradient-Vector Estimation" in chapter 5 in Adaptive Filtering Theory by SIMON HAYKIN, Prentice-Hall, 1986, and "Adaptice Transversal Filter using Recursive Least Squares" in chapter 8 of above book, respectively.
FIG. 1 shows a conventional block diagram of a receiver for mobile communication having an equalizer. A receive signal received by a receive antenna 61 is applied to a mixer 63 through a radio frequency amplifier 62. The mixer 63 carries out the multiplication of the receive signal and the local frequency from the local oscillator 64, and provides an intermediate frequency, which is applied to a quadrature detector 67 through a bandpass filter 65 which restricts the bandwidth of the receive signal, and an intermediate frequency amplifier 66. The quadrature detector 67 which has a local oscillator 71, a phase shifter 72 of /2, and a pair of mixers 73, provides a pair of baseband analog signals of in-phase element and quadrature element. Those elements are applied to the analog/digital converters 76 through the low-pass filters 75. The A/D converters 76 carry out the sampling and the quantization so that the input signal is converted to a digital form, and the converted digital signal is applied to an equalizer 77, which provides an equalized final output at an output terminal 78.
FIG. 2 shows a block diagram of a conventional decision feedback equalizer of a symbol tap spacing type for a mobile communication receiver.
The decision feedback equalizer comprises a feedforward transversal filter 84 for accepting an input signal IN from an output of the A/D converter, having a plurality of delay elements (T) 81, a plurality of multipliers 82 coupled with an input or an output of the delay element, an adder 83 for providing sum of outputs of the multipliers 82; a feedback transversal filter 85 which receives the output signal OUT of the equalizer, having a plurality of delay elements (T) 81, a plurality of multipliers 82, and an adder 83 for providing the sum of outputs of the multipliers 82; an adder 86 for providing the sum of two transversal filters 84 and 85; a decision circuit 87 which decides (1 or 0) the output y(i) of the adder 86, and provides an output signal OUT of the equalizer; an error detector 88 which compares the output signal OUT with the output y(i) of the adder 86 to provide an output error e(i) of the equalizer; a training memory 89 which stores a training signal for the initialization of the equalizer; and a tap coefficient calculator 90 which calculates the tap coefficients W(i) of each multipliers 82 by using the output error e(i), and input/output signals x(i+.alpha.) and d(i-.beta.) of each delay elements 81. The switch 91 in the figure switches the operation mode between the training mode and the tracking mode. In the training mode, the output of the training memory 89 is applied to the error detector 88 so that the output error e(i) is the difference between the output of the adder 86 and the output of the training memory 89. In the tracking mode, the output signal OUT is applied to the error detector 88 so that the output error e(i) is the difference between the output of the adder 86 and the equalized output signal OUT.
The equalizer of FIG. 2 operates first in the training mode in which the tap coefficients of the multipliers 82 are initialized by using a known training signal, and next in the tracking mode the equalization operation is carried out by using the initialized tap coefficients.
FIG. 3 shows the characteristics of the equalized output signal wherein an input signal is QPSK modulation signal, and the operational algorithm of the tap coefficient calculator in the equalizer is RLS algorithm. FIG. 3 shows the phase and the equalized output in the form of a projection on the Q-axis. FIG. 3(A) shows the case where the change of the characteristics of the propagation path is small, and it is found that the equalized signal is kept to that of the QPSK in both the training period and the tracking period, and the tracking characteristics are satisfactory. FIG. 3(B) shows the case where the change of the characteristics of the propagation path is large. In this case, the tracking characteristics are satisfactory only at the beginning of the tracking period, but the operation can not follow the change of the phase shift caused by the propagation path and the equalized phase becomes out of that of the QPSK signal.
Further, when there is some frequency offset between carrier frequency of a transmitter and local frequency of a receiver, the tracking characteristics in an equalizer are not satisfactory.
FIG. 4 shows the curve of a bit error rate of an equalizer when a frequency offset exists. In FIG. 4, the horizontal axis shows frequency offset in Hz, and the vertical axis shows bit error rate. It is found in FIG. 4 that only 300 or 400 Hz of frequency offset deteriorates the bit error rate characteristics considerably.
It should be noted that the amount of the frequency offset would be larger than several kHz in an actual communication circuit, and therefore, the compensation of the frequency offset is important.
Conventionally, a frequency offset is compensated by using a PLL (phase lock loop) which is coupled with an output of an equalizer. However, a prior PLL circuit has the disadvantage that it takes long time for locking-in the initial phase. If we try to increase gain of feedback loop of a PLL circuit to shorten the lock-in time, the operation of the PLL circuit would be deteriorated by the noise. Therefore, an equalizer which operates excellently, even when a frequency offset is large, must be developed.
Still another disadvantage of a prior art is as follows.
In a mobile communication, the propagation conditions, including absolute delay time of receive signal determined by the distance between transmitter and receiver, time difference of receive time of fast wave via the shortest propagation path and delayed wave via the roundabout propagation path, and absolute amplitude of rapid wave and delayed wave, change always reflecting the movement of a receiver.
As for the change of absolute delay time, a stable frame synchronization signal must be established, however, as it is difficult, a practical apparatus absorbs the change of the frame synchronization signal by increasing the number of taps of an equalizer.
Also, when time difference of receive time of fast wave and delayed wave is large, and/or when the state of propagation path is a non-minimum phase condition, the number of taps of an equalizer must be increased so that the residual inter-symbol interference is decreased.
On the other hand, the number of taps of an equalizer may be small, when a propagation path generates no delayed wave.
Conventionally, the number of taps of an equalizer is fixed so that the satisfactory communication quality is obtained even in the worst propagation condition. However, an equalizer having a large number of taps has the disadvantage that the calculation amount for the adaptive algorithm of the tap coefficient is increased, and the tracking characteristics following the change of the propagation path are deteriorated. In other words, an equalizer having a fixed number of taps has the disadvantage in the tracking characteristics.
Still another disadvantage of a prior art is as follows.
When the change of the propagation path is large, an adaptive algorithm can not follow quickly the change. In order to solve this problem, conventionally, the length of a transmission signal in a frame is shortened as compared with that of a training signal. Another prior solution is to insert a plurality of training signals in each burst.
However, above conventional solutions have the disadvantage that the transmission efficient is decreased. Another prior solution for a prior RLS algorithm is the use of a forgetting factor for exponential weighted RLS algorithm, but it has the disadvantage that the operation becomes unstable.